1. Field of the Invention
This invention relates to dispensing systems and, more particularly, to a computer-controlled, small quantity dispensing system for depositing controlled amounts of viscous fluids on a work piece employing a linear actuator operated piston.
2. Description of the Prior Art
Accurately controlled depositation of small quantities of viscous material on work pieces is an important part of the manufacturing process of many types of devices such as circuit boards and micro circuits. Such materials may include solder paste, adhesives, various epoxies, electrically conductive materials, potting materials, insulating materials and the like. Accurate small quantity, high speed deposition is also important in the medical fields for reagents, collegians, DNA samples and other fluids. While various systems and devices have been proposed for dispensing such materials, including drop-at-a-time dispensers, one successful approach has utilized the combination of a pressurized supply of material to a dispense chamber through which a piston moves to force predetermined quantities of material into a dispensing orifice. Such devices are shown, for example, in U.S. Pat. Nos. 4,572,103 and 4,941,428, the teachings of which are expressly incorporated herein by reference.
In such devices, the material to be dispensed is introduced to a container which may be pressurized by, for example, a supply of compressed air and which communicates via a passageway to a pre-dispense chamber positioned above and in communication with a needle-like dispensing orifice member. A piston is mounted for movement through the pre-dispense chamber into a pumping chamber open to the pre-dispense chamber and the dispensing orifice member. Movement of the piston into the pumping chamber forces material in the pumping chamber to move through the orifice member whereas withdrawal of the piston from the pumping chamber into the pre-dispense chamber permits a re-supply of material into the pumping chamber due to the pressurized supply from the container.
Such dispensing devices may have the piston actuated by means of, for example, a compressed air or hydraulic cylinder actuator. Adjustment of the amount of material to be dispensed per stroke of the piston can be made by a stroke limiter attached to the piston which can be mechanically adjusted to limit the stroke length by providing a fixed stop affixed to the piston, or to the piston actuator, or by changing the piston, pumping chamber and dispensing orifice for one of a different size.
While such devices have proven successful, they are limited in their versatility. Volumetric dispensing modifications must either be made by adjusting the stroke length of the piston manually or by replacing the piston, pumping chamber and orifice, either method creating disadvantages. Adjustments of the stop position require that the dispensing apparatus be interrupted in its cycle to allow access to the adjustment. Additionally, the adjustments are difficult to make with accuracy since they rely upon a manual repositioning of the stop and further may require many test runs before the optimum dispense volume is arrived at. Because it may be desirable to dispense different quantities at different points on a single work piece, such manual adjustment mechanisms are disadvantageous. Substitution of pistons and pumping cylinders is likewise time intensive and difficult and may involve disassembly of substantial portions of the device, a procedure that is detrimental to optimization of production runs.
It would, therefore, be an advance in the art to provide improved methods and structure for modification of the dispense volume of pressurized small quantity pump dispensers which allow rapid modifications to dispense volume to be made.
It would be a further improvement in the art to provide methods and devices allowing dispense volume modification to be made “on the fly” during operation of small quantity pre-pressurized piston pump dispensers.
It would be a further improvement in the art to provide for ease of removal of the piston and pumping chamber of a small volume, pre-pressurized piston pump dispenser. These and other improvements are obtained by the following described invention.